Question

Compare and contrast the reactivity of alpha hydrogens on carbonyl compounds and alkenes?

Answer 1

Well, you know that the electronegativity of oxygen is greater than that of carbon. That is basically the starting premise here.

As a brief review, electronegativity is the extent to which electron density is shared between atoms.

The larger the electronegativity difference, the more polarized the electron density is towards the more electronegative atom.

(Color key: Red is negative, blue is positive)

CARBONYL BONDS

So, a `"C"="O"` bond is more partially negative towards the oxygen and more partially positive towards the carbon.

Thus, a carbonyl carbon is electrophilic (it can get attacked), but the oxygen is nucleophilic (it can grab a proton). Because of this tendency for uneven sharing of electron density, oxygen is more capable of storing it than carbon and can easily grab a proton.

If oxygen does grab a proton, there is a greater partial positive charge on oxygen than without a proton.

That increases the tendency of oxygen to steal electron density from carbon to stabilize itself (because carbon, being more electronegative than hydrogen, has more electron density around it to take).

So, the carbonyl carbon becomes more electrophilic and thus more reactive with respect to nucleophilic attack.

REGULAR ALKENE BONDS

On the other hand, a `"C"="C"` bond has exactly even electron sharing between carbons (all other variables ignored, like electron-withdrawing groups, etc).

With this brings no greater capability to store electron density on either carbon than the other, except in cases of higher substitution on one carbon than the other.

Thus, when electron density is donated away from the `"C"="C"` bond to make a new bond, a carbocation is generated which must be stabilized.

ACIDITY OF ALPHA-HYDROGENS

The alpha-hydrogen (`"H"_alpha`) is right here:

Recall from above that we said oxygen holds the greater electron density in the `"C"="O"` bond, making the carbonyl carbon partially-positive.

Now, that electropositive carbon wants to stabilize itself because it is not "owning" enough of the electron density that it needs to bond (and bonding is favorable!).

So, it takes some from the alpha-carbon (`"C"_alpha`). In a bit of a "chain reaction", the `"C"_alpha` takes some away from the `"H"_alpha`. The less electron-sharing, the weaker the bond.

Thus, this weakens the `C_alpha-H_alpha` bond, increasing its acidity!

On the other hand, since the electron density is evenly shared in an ideal `"C"="C"` bond, there is no need to steal electron density away from anything else, so the `"H"_alpha` retains its share of electron density.

So, the alkene's `C_alpha-H_alpha` bond is not weakened, and thus, the `C_alpha-H_alpha` bond on the carbonyl compound is more acidic.

(How do we know? The pKa of the `"H"_alpha` on acetone is `20`, while the pKa of allyl hydrogens, `"C"="C"-"C"-color(blue)("H")`, are `~36`. So acetone is indeed more acidic.)

REACTIVITY CONCLUSIONS

As a result of the above discussion, we can conclude that:

• The `"H"_alpha` on a carbonyl compound is generally going to be plucked off by a strong-enough base (and weak-enough nucleophile).

• If the nucleophile is strong enough for the given situation, the carbonyl carbon is going to be attacked.

(Water is only strong enough because the oxygen was protonated.)

• For the alkene, neither carbon is a greater target to nucleophilic attack, and thus either carbon can take the partial positive charge once the `"C"="C"` bond breaks, requiring the carbocation intermediate to stabilize itself with the surrounding `"C"-"H"` bonding electron pairs (review hyperconjugation!).